The present invention relates to the field of vehicle transmission systems, and more particularly relates to the field of automatic control of secondary transmission systems which are mounted to vehicles in series with primary transmission systems. In particular, the present invention relates to a control system for a sub-transmission, of a vehicle, which enables good engine braking to be obtained while driving the vehicle.
It has been conventionally well known, and practiced, to provide a vehicle, which is equipped with a primary transmission system which is changed over by hand, with a secondary transmission or sub-transmission mounted in series with the main transmission, said sub-transmission being changed over between two gearing ratios by hand, independently of said primary transmission. However, although such a combination has the advantage that, by it, many steps of speed change, i.e. many overall gearing ratios, can be provided, and although thereby it is possible to operate the vehicle so as to obtain good fuel economy and high smoothness of running of the vehicle, it has the disadvantage that operation by hand of both the primary transmission and the sub-transmission is a difficult process. The proper synchronization of the operation by the driver of a hand gear changing mechanism for the primary transmission and of another hand gear changing mechanism for the sub-transmission is time-consuming, may distract the driver's attention from road conditions, thus perhaps even causing a serious accident, and further may not be performed adequately at a crucial moment, due to the pressure of circumstances requiring the driver's attention, thus resulting in inefficient or unsmooth operation of the vehicle. Thus, if this system is to be used, the driver must be specially trained.
To solve this problem, it has been formerly proposed to provide a sub-transmission gear changing system which is automatically and electrically controlled, not depending directly upon the will of the driver, but according to various combinations of operating parameters of the vehicle and of its engine, so that the driver may only control manually the operation of the primary or main transmission, and the operation of the sub-transmission is performed automatically between its high and its low gearing ratios.
In a vehicle which is equipped with a manual transmission, it is often very helpful for driving the vehicle to take advantage of so-called engine braking by, at the appropriate time, downshifting the transmission, so that, when the vehicle is decelerated, a larger part of the decelerating impetus is provided, not from the braking system of the vehicle, but from overrunning the engine of the vehicle, so as to operate it as a pump at a higher rate and thereby dissipate a larger amount of energy of motion of the vehicle in friction and hydraulic compression losses. Such a form of deceleration is particularly valuable in wet or icy road conditions, because of its progressive and anti-skid nature. However, it has been a disadvantage up until now of the above outlined system of controlling a sub-transmission automatically that, when the vehicle is decelerated, sometimes engine braking effect is not good; indeed, due to closing of the throttle of the vehicle during braking, sometimes the sub-transmission may even shift to its higher gearing ratio, thereby actually diminishing the engine braking effect. Therefore, up until now, good engine braking effect has not been obtainable from a vehicle equipped with a manual transmission and an automatically controlled sub-transmission, and this has been a significant reason militating against their acceptability to the general public.